Properties of Neutron Stars with Hyperons within a Relativistic Metamodel

TL;DR

The paper develops a relativistic metamodel for the neutron-star equation of state that includes Λ-hyperons and explores two coupling strategies, SU(6) and Ratio, within a Bayesian framework constrained by χ-EFT, AME2016, hyperon potentials, GW170817, and NICER observations. It finds that hyperons generally soften the EOS and lower the maximum mass, but the extent of radius changes depends on the chosen hyperon couplings; relaxing SU(6) (Ratio) broadens the allowed NS properties and brings hyperonic and nucleonic mass–radius sequences into closer agreement under current constraints. The study also compares two nucleonic functionals (GDFM and TW), showing that the underlying nucleonic model significantly affects radii and the degree of hyperon impact, while stability analyses indicate no spinodal instabilities within realistic parameter ranges. Overall, the work provides a Bayesian, model-flexible pathway to quantify hyperon-induced uncertainties in NS observables and demonstrates that hyperons can be compatible with present multimessenger constraints. The framework offers a practical route to interpret future GW and X-ray data in the presence of strangeness in dense matter.

Abstract

In this work, we study the effects of $Λ$-hyperons on neutron star properties employing a metamodel framework for the equation of state (EoS). Different choices for defining the hyperonic couplings with different levels of parametric freedom are discussed. In all models, the predicted NS maximum masses are reduced compared with the purely nucleonic composition as expected. In the case of relating hyperonic couplings via $SU(6)$-symmetry arguments to the nucleonic ones, we find that NS radii for intermediate mass stars are shifted to higher values compared with purely nucleonic stars, in agreement with the existing literature. However, allowing for more freedom for the hyperonic couplings, the effect is strongly reduced, and the distributions in the NS mass-radius plane of models with and without hyperons become very close. We have also investigated how different nucleonic density functionals influence the hyperon matter composition and neutron star properties.

Figures (11)

• Figure 1: Pressure, proton fraction, $\Lambda$ fraction as a function of baryon density $n_B$, and mass-radius for three central EOS models from the distributions for each cases, respectively, see text for details.
• Figure 2: Contours of pressure at $\beta$-equilibrium as function of baryon number density at different CI for the Ratio and $SU(6)$ cases.
• Figure 3: Same as Fig. \ref{['fig:p-rho']}, but for speed of sound.
• Figure 4: Same as Fig. \ref{['fig:p-rho']}, but for proton fraction.
• Figure 5: Same as Fig. \ref{['fig:p-rho']}, but for $\Lambda$ fraction.